Infrared thermography systems are used in particular to visualize at least a portion of the countryside by means of infrared radiation in the frequency spectrum at a wavelength longer than one micron. Visualization in the infrared range can take place day or night, but is generally more useful at night when direct vision in the visible range is reduced or impossible. The spectral bands of the radiation detected by these thermographic systems correspond to atmospheric transparent windows, at wavelengths of either 3 to 5.mu., or 8 to 12.mu.. The latter spectral band is particularly suited to thermographic systems because it is perfectly adapted to imaging bodies at ambient temperature, because a black body at 300.degree. K has its maximum emission at about 10.mu.. Systems that employ optical/mechanical raster and line scanners are generally known, as discussed in La Thermographie Infrarouge [Infrared Thermography] by G. GAUSSORGUES, Part 7, published by Technique et Documentation II rue Lavoisier 75384 Paris Cedex of France on Dec. 2, 1980, herein incorporated by reference.
The result of an operation involving sky, land or sea horizon surveillance, by an infrared thermography system is that a fixed or moving object that is worthy of interest is detected. The camera's angle of field is of the order of only a few degrees when observation distances are of the order of 10 km. This corresponds to a very small displayed image of an object where the detected object is only of the order of a few meters in diameter. In such instances it is desirable to enlarge the image portion which contains the object and thus facilitate viewing. This is preferably accomplished by displaying the enlarged image over the entire television monitor screen using an electronic enlarger or a full-screen electronic enlarger.
Analog processing has been used for several years to move images on the screen. When the signal from the camera to the television monitor is subjected to analog processing, electronic enlargement is accomplished by expansion of scanning to visualization, i.e., by expansion by the same ratio of similarity that exists on the abscissa and ordinate axes for the electron beam in the image tube of the television monitor. The principal drawbacks of this technique are a reduction in screen luminosity and a doubling of line spacing.
Generally, it is preferable to represent the camera output signals in digital format, because this yields greater signal processing flexibility. The present invention is directed to an infrared thermography system in which the camera output signals are digitized and in which a parallel, or series-parallel, raster scanner is used. The raster scanner can be either a single raster or preferably interlaced even and odd rasters. Field scanning accomplished by infrared detector scanning, or by series scanning using a bar of several detectors, requires a line scanner which operates at a very high operating speed thus giving rise to various synchronization problems, and also requires a very fast and sensitive photodetector. In order to eliminate these technical constraints, the preferred practice is to simultaneously scan m lines of the field of view using a detection matrix having m detectors in the raster direction, thus making it possible to divide line scanner speed by m while enabling the movement of raster scanning to remain unchanged. This latter system constitutes scanning by parallel scanning, and furthermore is compatible with scanning by series scanning, with series-parallel scanning by a two-dimensional mosaic or infrared detectors moreover being of the most interest. On this subject, see the work by G. GASSORGUES, cited above, pages 244 to 248.
However, regardless of the scanning mode adopted to scan the field, signal digitization permits copying into storage. This storage capability provides the advantage of enabling an electronic digital enlarger to be realized. The storage of a network of digital points, 6-bit codes, for example, which represent a sampled image, in an image storage device, or in a raster storage device, is known. Starting with these sample points, intermediate points can be fixed by interpolation, in both the line direction and the raster direction at the same time, using filtering techniques, preferably a digital filtering technique. Filtering is even more developed than interpolation when consideration is given to the greater number of sample points in the vicinity of the image that must be recreated. The simplest filters duplicate the closest sample points, which corresponds to/cos/filtering, or interpolate only from the two closest sample points, which corresponds to increased cosine filtering. An X2 digital enlarger, for example, requires three image points to be recreated per sampling point, and an X4 digital enlarger requires 15 image points per sampling point. This latter technique mitigates the problems associated with analog electronic enlargers as discussed above, i.e., luminosity decrease and information density on the television monitor screen, but at the same time has its own drawbacks. Generally, this technique requires a large storage capacity, corresponding at a maximum to one-half the field height in the raster direction, if full-screen digital enlargement is desired. This technical problem can be resolved in part by a compromise that consists of enlarging only the portion of the field containing the object of interest, which has been previously detected with the system operating in its customary mode and toward which the camera had been oriented in advance. However, this solution is not entirely satisfactory, although generally it is preferable to achieve full-screen digital enlargement. Moreover, even assuming that full-screen digital enlargement is achieved using this latter technique along with a large-capacity storage unit, system operation is clearly not optimum because three-fourths of the information available at the camera output is still not used, and this can be represented in the form of a loss to gain equal to a signal/noise ratio of .sqroot.4.
There is yet another drawback that arises in the case of interlaced raster operation, e.g., using two even and odd interlaced rasters. Prior to recreating an image by interpolation of sample points, it is necessary to store in advance all raster image information that can, in case of movement due to displacement of the camera and/or to movement of the object in the field, cause breakup in the outline of objects. It is also necessary to obtain calculated intermediate points that are relatively false because they were obtained from adjacent points from different rasters that to all intents and purposes no longer have any correlation with each other, particularly in the case of high spatial field of view frequencies. Furthermore, when operating with interlaced rasters, it is current practice to work out one vertical being sampled in each raster because the eye, which itself functions as a filter, perceives a sampling that is sufficient for the complete image, that is, in the case of interlaced rasters that can present the phenomenon of spectrum overlapping when taken individually. After regular successive interlacing as a function of time, the spectrum overlapping phenomenon disappears except for a moving scene presenting high spatial frequencies. With the known digital enlarger described above, interpolation between adjacent points is not significant because the rasters processed contain adjacent lines that are not correlated as a function of time and appear in different rasters during scanning.